For most of sheet-making processes, an online quality measurement and control system (QCS) is an equipment useful to achieve intended sheet qualities and to optimize production throughput. In a QCS, one of the mechanical components of interest is a scanning apparatus 100 that allows multiple online sheet quality measuring sensors to be mounted thereon. Scanning apparatuses 100 are typically constructed as a rectangular structure where top and bottom beams 112, 114 are supported with two end columns 116, 118 as shown in FIG. 1. Sheet material 80 can travel through an open space between top and bottom beams 112, 114.
Online sheet quality measuring sensors typically include two modules, one module 130 including a source is mounted on a carriage 330 movable on one beam 114 and the other module 140 including a detector is mounted on another carriage 340 movable on the other beam 112. During production, the sheet material 80 or continuously produced sheet material 80 hence travels between two sensor modules 130, 140 which are carried by the top and bottom carriages 330, 340, respectively. Usually a constant source signal is transmitted from the source module 130 through the moving sheet material 80 and is detected by the detection module 140. A variation in the detected signal can be used to determine sheet quality variations. In this setup, the top and bottom carriages 330, 340 are synchronized and jointly traverse back and forth in a cross direction CD for scanning 50 between sheet edges 81, 82 while the sheet material is moving through a sensing gap 150 between the source module 130 and the detection module 140. A system 400 takes the detected signal from the detection module 140 and the carriage moving position to build scanning measurement 401 as a function of cross direction CD position. The scanning measurement 401 can also be referred to as profile.
In order to achieve high precision sheet quality measurements, it can be ensured that the source module 130 and the detection module 140 are aligned precisely while they are traversing back and forth 50 in a cross direction CD and, e.g., perform a forward scan 51 and/or a reverse scan 52. The alignment between the source module 130 and the detection module 140 can be obtained with a number of ways. For instance, mechanical dial indicators or magnetic alignment sensors can be mounted on the top and bottom carriages 330, 340 to detect misalignments, also known as deflections D, between the top and bottom carriages 330, 340 and/or between the source module 130 and the detection module 140. The detected deflection D can be used for diagnostics of conditions of mechanical components or operations of the scanning apparatus 100, e.g., to control carriage movement, and/or to compensate the impacts of deflection on sensor measurements.
However, in practice, mechanical dial indicators are bulky, fragile and difficult to mount, and adding magnetic alignment sensors to an existing scanning apparatus 100 may not be a trivial upgrade. Adding new magnetic alignment sensors like Hall-effect sensors or their equivalent usually involves various degrees of mechanical alternation to a scanning apparatus 100 and/or significant software changes that may or may not be even feasible for older QCS systems.
In reality, there are tens of thousands of scanning apparatuses that have been installed and continue being in use around the world in the past several decades. Even though some older scanning apparatuses may have been shut-down, upgraded or replaced, there are still large number of different generations of scanning apparatuses currently in operation. From a practical point of view, all scanning apparatuses in operation will need regular check-up of their mechanical conditions in order to provide solid foundation for online sheet quality sensors to get high precision measurements.
Therefore, there is a need to have an easy yet effective way to perform scanning apparatuses check-up and related services.